This invention relates to a method of filtering metal contaminants from a lubricating coolant and for recycling a constant volume of filtered lubricating coolant to a workpiece to be machined, and more particularly to a method of reducing the effects of fluctuating fluid pressure differential between the inlet and the outlet of a filter unit incorporated into such a system.
Coolant filtration systems have been employed for many years in the machine tool industry providing the essential function of servicing used, metal-contaminated lubricating coolant to remove metal contaminants therefrom and for recycling filtered lubricating coolant to the machine station for subsequent use.
At the machine station, it is critical to provide filtered lubricating coolant at preselected volume flow and pressure in order to properly cool and lubricate the tool bit as well as the workpiece which is to be machined. Any variation from the preselected values of coolant flow and pressure may result in costly damage to the machine tool, and perhaps more importantly, cause irreparable distortion of the workpiece due to excessive heat build-up. Since precision must be maintained at all times during the machining operation in order to meet demanding tolerance criteria, the necessity of providing proper lubricating coolant pressure and flow becomes readily apparent.
One typical prior art arrangement for a lubricating coolant filtration/recycling system is comprised of a contaminated coolant receiving vessel, a pump for pumping contaminated coolant from the receiving vessel, a filtration unit positioned downstream of the pump, a clean tank for receiving and accumulating filtered coolant from the filtration unit, and a clean pump positioned downstream of the clean tank for pumping the accumulated filtered coolant to the machine station.
The function of the clean tank is multifold. Primarily, it provides a reservoir of filtered coolant which the clean pump may draw from. In addition, the clean tank acts to dampen fluctuations in volume and pressure of filtered coolant exiting the filtering unit. These fluctuations are the result of pressure differential changes occuring between the inlet and the outlet of the filtration unit due to accumulation of contaminants upon the filter surfaces, as well as to changes in fluid pressure differential across the filtering unit while individual filters of the filtering unit are being serviced, for example, during backwashing.
This dampening effect causes a reduction in the occurence of fluctuations in filtered fluid volume flow and pressure when supplied by the clean pump to the machining station.
As was pointed out earlier, the provision of constant volume flow and pressure of filtered coolant at the work station is critical to the machining operation.
In addition, the clean tank must be selected so as to have a capacity to accumulate filtered coolant in excess of that required by the clean pump. The excess capacity of the clean tank varies greatly depending upon various operating parameters relating to the filtering unit as well as to the feed pump. These operating parameters will become more apparent as a result of the following discussion of the relationship between the filtering unit and the contaminated fluid feed pump.
All filtering units are designed to retain particles above a certain size. This results in a fluid pressure differential across the filtering unit, with higher pressure resulting at the inlet of the filtration unit and lower pressure resulting at the outlet of the filtration unit.
There are two major factors which affect the magnitude of the fluid pressure differential. The first is flow rate through the filter. As the flow rate through the filter increases, the fluid pressure differential increases. The second factor is contaminant loading on the filter surface. As the contaminant loading increases, the fluid pressure differential increases. However, as contaminent loading increases, flow rate decreases.
Another well known characteristic of filtering units is the relationship between fluid pressure differential and time. For example, it takes a relatively long period of time to reach a fluid pressure differential of 20 PSI as compared with the time required to go from 20 PSI to 30 PSI. Therefore, it is usually desirable to clean the filter elements somewhere in the area of this marked transition period.
The filter feed pump is typically of a centrifugal type of well known design. The benefits of this type of pump include its low initial cost, solid handling capacity as well as its relative efficiency. However, centrifugal pumps have a significant drawback in that small changes in the resistance in the discharge line (filtering unit) downstream of the pump result in large changes in the flow volume exiting the pump.
Returning now to the relationship between the filtering unit and the filter feed pump, it becomes apparent that in order to maximize the efficient use of the filtering unit, it should be allowed to reach a fluid pressure differential of, for example, 20 PSI. However, that same 20 PSI pressure differential change may cause a change in the flow rate of up to twice that required at the machining station.
Furthermore, from the minimum flow rate as required at the machining station to the maximum flow rate in the operating range of the pump, there may be far less than 20 PSI fluid pressure differential thereby reducing the effectiveness of the filtering unit.
Finally, since the size of the filtering units are based upon flow rate, if the maximum flow rate occurs at minimum fluid pressure differential, the filter may need to be twice as large as that needed for filtering only the required volume of coolant actually called for at the machining station.